The invention relates to a method for monitoring and positioning a beam or jet for operating on a workpiece, in which a first sensor (for example, of a seam detecting system in the case of a beam) ahead of the beam or jet, and/or a preset value, determine(s) the path to be followed by the beam or jet, and a second sensor behind the beam or jet monitors the action of the beam or jet. The invention also relates to an apparatus for carrying out the method.
In a large number of primarily industrial, machining processes a beam or jet has to be monitored and/or positioned. This is true in laser beam cutting or water jet cutting, for example. In both cases, the path of the beam or jet is predetermined, usually by an electronic system. The beam or jet then has to follow this path. Cutting quality depends upon (among other factors) the precise adherence of the beam or jet to the predetermined route.
To an even greater extent the same is true in welding with a beam, and especially in laser beam welding. Again, the very advantages of sheet-metal parts mass-produced by butt welding by the laser beam process (eg. those known in the motor vehicle industry as "tailored blanks") cannot be fully exploited unless the exacting requirements of this process, in terms of geometrical tolerances of components, quality of cut edges, and characteristics of the laser radiation, are met.
With a normal focus diameter of 0.2 to 0.4 mm, exact positioning of the laser beam on a joint line between the two workpieces to be joined together is essential in order for a high-quality welded joint to be obtained. For a technical "nil" gap, the maximum position tolerance should not exceed 0.1 mm. Moreover, where gaps have to be bridged by the molten pool, this tolerance must be further reduced.
Lack of fusion due to poor beam positioning is particularly critical in welds with only partial penetration, being concealed by the wider upper run of weld. Even in welds with full penetration, a slightly widened seam root may result in invisible internal lack of fusion.
Sufficiently high positioning accuracies are obtained nowadays by means of optoelectronic sensors or image processing systems which transmit the actual track of the joint line, ie. the track of the abutting edges of the workpieces, ahead of the beam, directly to the machine control system as a series of correction coordinates. By these means, a laser head can be orientated with respect to the seam with accuracies of approximately 0.05 mm. A seam detecting system of this kind is disclosed for example in DE-OS 4312241.
A fundamental drawback of these methods lies in the fact that the joint line detection systems have to start out from a fixed preset laser beam position. Measured position coordinates cannot be referenced to the actual position of the beam. This is partly remedied by carrying out a calibration procedure at periodic intervals, and as a minimum after each adjustment of the optical path, but this involves a break in production. Changes in position of the laser beam due to thermal effects in the beam generation and guidance system remain wholly uncompensated. The result is a reduction in the precision of the welding operation.
Theoretically these problems could be solved by incorporating a beam position sensing system into the beam guidance system already present as part of the beam detecting system. However, such sensing systems are usually unsuitable for on-line operation and, at the very least, their use is likely to incur considerable additional cost.
Furthermore it is known that a welded seam can be monitored behind the beam. However, this is done not in order to take account of the position of the joint line, but merely to determine the quality of the welded seam.